CN0287 accuracy analysis

Hi,

One of our new designs, requires us to implement RTD & Thermocouple measurement circuit.

We need to get a accuracy of better than "0.2% of FSR" over the operating temperature range of "-20°C to 60°C".

We have looked at CN0287 board & planning to use the same circuit in our board if the required accuracy is met.

The CN0287 specification says that accuracy of 0.5°C. Is this accuracy guaranteed over a specific temperature range?

For our accuracy analysis , we have identified the error introducing components as below :

RTD measurement :

  • Resistor Rref  - Tolerance & Temperature coefficient
  • ADC AD7193 - ENOB , INL , Offset error, Gain error , Offset drift , Gain drift

Due to ratio-metric cancellation,  change in excitation current & ADC reference voltage cancel out each other. Thermo-couple measurement :

  • ADC AD7193 - ENOB , INL , Offset error, Gain error , Offset drift , Gain drift
  • ADC Reference ADR3425 - Initial Accuracy, Line regulation , Load regulation, Output Noise
  • Cool Junction Compensation ADT7310 - Accuracy ( 0.5°C )

Please let us know if we have missed out any parameter in our analysis.

Also, it will be helpful if you can share any document related to accuracy analysis done on CN0287.

Looking forward to your response.

Regards,

Suresha N S

Parents
  • 0
    •  Analog Employees 
    on Dec 12, 2013 8:36 AM

    Hello Suresha N S

    For this circuit, we assume customer doing the system calibration to cancel out all the linearity error. Besides that, the non-linearity error (drift, noise) would decide the final accuracy performance. We only tested the circuit under room temperature and ignoring the drifting effect. By testing, we find that the noise performance which could decide the final resolution, is good enough to satisfy the system accuracy requirement. The real bottleneck  to get better accuracy comes from ADT7310 with 0.5°C accuracy which is used for CJC in this circuit without considering drift.

    For general analysis, first, you need to find the dominated error items in your circuit. For CN0287, the dominated error items for thermocouple circuit is from 1st: voltage reference, 2nd: Accuracy of CJC. The both error items effect the final system accuracy directly. For 80°C working temperature range, ADR3440(8ppm) will generated 640ppm = 0.064%. Assuming your signal covers 50% of dynamic range of ADC by configuring the gain block in AD7193 properly, the final effect by reference is 0.13%. For -200~800°C measurable range, the accuracy would be 1.3°C. Use square root of 1.3 and 0.5 = 1.4°C.

    In summary, the system accuracy limitation would come from ADR3440 if considering the 80°C temperature changing.

    For CN0287 using RTD sensor, the main error items would be the reference resistor with 10ppm temperature coefficient. So, 800ppm for 80°C temperature difference. RTD has common mode voltage that limit the gain of the signal chain. For -65°C~250°C, the resistance of PT1000 would be 763.3 Ω to 1941 Ω. With 4.02K reference resistor, actually, we could set the gain to 2 but this is not available for AD7193. The only available gain would be 1. So that (1941-763.3)/4020 ≈30%. The final system accuracy would be around 800ppm/30% = 2400ppm = 0.24%.

    So you need to use reference resistor with 5ppm to meet your system performance.

    Also, other error items such as gain drift, offset drift, leakage would effect the final performance as well. You need to do more calculation to get closer to the real result but the real testing result beats all the calculation.

    And some times real testing will be more easier than calculating.

    Wish you all the best.

    With best regards

    Willie.

Reply
  • 0
    •  Analog Employees 
    on Dec 12, 2013 8:36 AM

    Hello Suresha N S

    For this circuit, we assume customer doing the system calibration to cancel out all the linearity error. Besides that, the non-linearity error (drift, noise) would decide the final accuracy performance. We only tested the circuit under room temperature and ignoring the drifting effect. By testing, we find that the noise performance which could decide the final resolution, is good enough to satisfy the system accuracy requirement. The real bottleneck  to get better accuracy comes from ADT7310 with 0.5°C accuracy which is used for CJC in this circuit without considering drift.

    For general analysis, first, you need to find the dominated error items in your circuit. For CN0287, the dominated error items for thermocouple circuit is from 1st: voltage reference, 2nd: Accuracy of CJC. The both error items effect the final system accuracy directly. For 80°C working temperature range, ADR3440(8ppm) will generated 640ppm = 0.064%. Assuming your signal covers 50% of dynamic range of ADC by configuring the gain block in AD7193 properly, the final effect by reference is 0.13%. For -200~800°C measurable range, the accuracy would be 1.3°C. Use square root of 1.3 and 0.5 = 1.4°C.

    In summary, the system accuracy limitation would come from ADR3440 if considering the 80°C temperature changing.

    For CN0287 using RTD sensor, the main error items would be the reference resistor with 10ppm temperature coefficient. So, 800ppm for 80°C temperature difference. RTD has common mode voltage that limit the gain of the signal chain. For -65°C~250°C, the resistance of PT1000 would be 763.3 Ω to 1941 Ω. With 4.02K reference resistor, actually, we could set the gain to 2 but this is not available for AD7193. The only available gain would be 1. So that (1941-763.3)/4020 ≈30%. The final system accuracy would be around 800ppm/30% = 2400ppm = 0.24%.

    So you need to use reference resistor with 5ppm to meet your system performance.

    Also, other error items such as gain drift, offset drift, leakage would effect the final performance as well. You need to do more calculation to get closer to the real result but the real testing result beats all the calculation.

    And some times real testing will be more easier than calculating.

    Wish you all the best.

    With best regards

    Willie.

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